Gas stream analysis using voltage-current time differential operation of electrochemical sensors

ABSTRACT

A method for analysis of a gas stream. The method includes identifying an affected region of an affected waveform signal corresponding to at least one characteristic of the gas stream. The method also includes calculating a voltage-current time differential between the affected region of the affected waveform signal and a corresponding region of an original waveform signal. The affected region and the corresponding region of the waveform signals have a sensitivity specific to the at least one characteristic of the gas stream. The method also includes generating a value for the at least one characteristic of the gas stream based on the calculated voltage-current time differential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/055,562, now U.S. Pat. No. 9,581,564, filed on Oct. 16, 2013, whichis incorporated by reference herein in its entirety.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

Increasingly stringent emissions regulations require automobilemanufacturers to develop comprehensive on-board diagnostic (OBD) systemsfor exhaust gas monitoring. Compact, inexpensive sensors areparticularly in demand for monitoring and control of regulatedpollutants including hydrocarbons, carbon monoxide, and oxides ofnitrogen (NO_(x)). Sensors for these applications have been proposedbased on semiconducting oxides, heterocontacts in semiconducting oxides,surface acoustic waves, and capacitance. Other sensors are based onsolid-state electrochemical devices, which typically use a solid ceramicelectrolyte attached with two or more metal or metal-oxide electrodesand operate in either potentiometric (open circuit) or amperometric(DC-biased) modes.

Significant progress has been made towards the development of deployablesensors using yttria-stabilized zirconia (YSZ) as the electrolyte ofsolid-state electrochemical devices. However, there still existsignificant shortcomings related to stability, sensitivity, responsetime, and cross-sensitivity. These shortcomings are responsible for onlyone type of NO_(x) sensor being available commercially to date. Thecommercially available amperometric NO_(x) sensors are not ideal forwidespread use due to high cost, complexity, and limited performance.The benchmark sensor is the well-known YSZ-based oxygen sensor currentlyused in almost all automobiles. Although that sensor demonstrates thecommercial feasibility of this technology, it addresses a task that isless complicated than low concentration (ppm level) gas sensing used inNO_(x) applications.

Some approaches for solid-state electrochemical sensors usefrequency-domain impedancemetric modes of operation. This approachrelies on specific material compositions and microstructures to maximizethe sensor response at higher frequencies, because higher frequenciesallow for faster sampling rates and improved signal-to-noise ratios.This approach also relies on the measurement of the phase angle as ametric for monitoring the gas concentration.

SUMMARY

Embodiments of the invention relate to a device for signal processing.The device includes a signal generator, a signal detector, and aprocessor. The signal generator generates an original waveform. Thesignal detector detects an affected waveform. The processor is coupledto the signal detector. The processor receives the affected waveformfrom the signal detector. The processor also compares at least oneportion of the affected waveform with the original waveform. Theprocessor also determines a difference between the affected waveform andthe original waveform. The processor also determines a valuecorresponding to a unique portion of the determined difference betweenthe original and affected waveforms. The processor also outputs thedetermined value.

Embodiments of the invention relate to a system for analysis of a gasstream. The system includes a signal generator, an electrochemicalsensor, a signal detector, and a processor. The signal generatorgenerates an original waveform. The electrochemical sensor receives theoriginal waveform. The electrochemical sensor is disposed at leastpartially within the gas stream. The electrochemical sensor includes afirst electrode and a second electrode. The second electrode is disposedrelative to the first electrode. The signal detector detects an affectedwaveform from the electrochemical sensor. The processor is coupled tothe signal detector. The processor receives the affected waveform fromthe signal detector. The processor also compares at least one portion ofthe affected waveform with the original waveform. The processor alsodetermines a difference between the affected waveform and the originalwaveform. The processor also determines a value corresponding to atleast one characteristic of the gas stream based on a unique portion ofthe determined difference between the original and affected waveforms.The processor also outputs the determined value corresponding to the atleast one characteristic of the gas stream.

Embodiments of the invention relate to a method for analysis of a gasstream. The method includes identifying an affected region of anaffected waveform signal corresponding to at least one characteristic ofthe gas stream. The method also includes calculating a differencebetween the affected region of the affected waveform signal and acorresponding region of an original waveform signal. The affected regionand the corresponding region of the waveform signals have a sensitivityspecific to the at least one characteristic of the gas stream. Themethod also includes generating a value for the at least onecharacteristic of the gas stream based on the calculated difference.

Embodiments of the invention relate to a method for temperature analysisof a gas stream. The method includes identifying a temperature parameterof an affected waveform signal. The method also includes calculating achange in the temperature parameter by comparing the affected waveformsignal with an original waveform signal. The method also includesgenerating a value from the calculated change which corresponds to thetemperature of the gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic block diagram of one embodiment of an exhaustsensor system.

FIG. 2A depicts a schematic diagram of one embodiment of a gas sensorfor use in the sensor assembly of FIG. 1.

FIG. 2B depicts multiple embodiments of sensor arrangements.

FIG. 3A depicts one embodiment of a waveform diagram of a triangularinput waveform and corresponding response waveform regions.

FIG. 3B depicts waveforms similar to the waveforms of FIG. 3A withadditional markings to indicate time gaps between 0 A and 0V on bothsides of the waveform.

FIG. 3C depicts the waveforms of FIG. 3B with an emphasis on a portionof the triangular waveforms.

FIG. 3D depicts the waveforms of FIG. 3B with the markings of FIG. 3Band additional markings at non-zero crossings.

FIG. 3E depicts a sawtooth input waveform with corresponding responsewaveform regions.

FIG. 3F depicts an embodiment of response signals corresponding tovaried gas species concentrations.

FIG. 4 depicts one embodiment of a flow chart diagram of a method formeasuring gas species using voltage-current differential in thetime-domain.

FIG. 5 depicts one embodiment of a flow chart diagram of a method formeasuring temperature using voltage-current differential in thetime-domain.

Throughout the description, similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments asgenerally described herein and illustrated in the appended figures couldbe arranged and designed in a wide variety of different configurations.Thus, the following more detailed description of various embodiments, asrepresented in the figures, is not intended to limit the scope of thepresent disclosure, but is merely representative of various embodiments.While the various aspects of the embodiments are presented in drawings,the drawings are not necessarily drawn to scale unless specificallyindicated.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by this detailed description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment of the invention. Rather, language referring to thefeatures and advantages is understood to mean that a specific feature,advantage, or characteristic described in connection with an embodimentis included in at least one embodiment of the present invention. Thus,discussions of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize, in light ofthe description herein, that the invention can be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments of the invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the indicatedembodiment is included in at least one embodiment of the presentinvention. Thus, the phrases “in one embodiment,” “in an embodiment,”and similar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

While many embodiments are described herein, at least some of thedescribed embodiments facilitate the detection of pollutant gases in ahot, flowing gas stream. In the course of investigating frequency-domainimpedancemetric modes of operation, a surprising discovery occurred whenemploying inexpensive digital electronics to monitor the voltage-currentdifferential in the time-domain. A source wave was applied to a sensorthat was subjected to a gas stream, and a corresponding response wavewas obtained. The source and response waves exhibited similarpeak-to-peak values indicating that no phase angle shift or phase angledifference occurred.

In contrast, previous impedancemetric modes of operation in thefrequency-domain used expensive electrochemical equipment that indicatedphase angle changes. Nevertheless, the distortion of the response wavein the low-cost digital electronics did result in a time-voltagedifferential at the zero-crossing, as well as at non-zero magnitudes.

Furthermore, using the time-voltage differential in the time-domainallowed for larger amplitude signals indicating the potential for highersensitivity toward NO_(x) and lower constraints for specified materialcompositions and microstructures to achieve different levels ofperformance. Thus, it was discovered that the use of time-voltagedifferentials in the time-domain allows for more sensor designflexibility.

A digital method for operating embodiments of solid-stateelectrochemical gas sensors using time-domain measurements may haveadvantages over conventional direct current (DC) methods such aspotentiometric and amperometric sensors, as well as other alternatingcurrent (AC) methods such as frequency-domain impedancemetric sensors.

In some embodiments, the applied signal is an AC waveform. The appliedsignal may be any type of symmetrical or asymmetrical AC waveform. Insome examples, particular waveforms such as sinusoidal or triangularwaveforms may be applied. In some embodiments, alternate waveform shapes(i.e., other than a sinusoidal waveform) may produce larger responsesignals.

In some embodiments, the response of the solid-state electrochemical gassensor is digitally measured as a voltage-current time differential asindicated by the time domain zero-crossing. In other embodiments, thevoltage-current time differential is monitored at another specifiednon-zero magnitude. In some embodiments, measurement of thevoltage-current time differential at specific points of magnitude otherthan the zero-crossing produces larger signals. Additionally, theapplication of a particular type of waveform, or a waveform withspecific characteristics, may influence the sensitivity or othercharacteristic of the response signal that is detected. For example, insome embodiments, the application of a triangular waveform may result inhigh sensitivities and large sensor signals.

In some embodiments, the digital time-domain method can be used forsimultaneous measurement of multiple gas species and/or environmentalvariations such as temperature. For example, in a single wave cycle, thesystem is capable of measuring multiple species of gases within anexhaust stream. As another example, the detection of part-per-million(ppm) levels of NO_(x) (e.g., NO and NO₂) as well as temperature inautomotive exhaust may be done through the application of bothtriangular and sine waveforms. In another example, asymmetric signalsand multiple voltage-current time differentials can be used to extractmeasurements of multiple gas species simultaneously. In yet anotherexample, combining changes in source wave frequency can be used toextract temperature information of the sensor and/or the gas stream.

In further embodiments, certain material and design features can beidentified specifically for detecting NO_(x) using a solid-stateelectrochemical sensor that are based on reaction mechanisms responsiblefor sensing. However, the potential applications for the embodimentsdescribed herein are significantly broader in terms of the types of gasspecies that can be detected (e.g., oxygen, nitrogen dioxide,hydrocarbons, etc.) and are also not necessarily limited to solid-stateelectrochemical gas sensors.

Development of this technology is of interest to various vehicletechnologies. And embodiments of this technology have primary,short-term applications for on-board monitoring of vehicle (especiallydiesel) emissions. However, while many embodiments described hereinspecifically refer to the monitoring of industrial exhaust gases andvehicle emissions, broader applications are available in any area whereelectrochemical sensors are of interest. For example, some embodimentsdescribed herein may be employed in medical, health & safety, andenvironmental applications.

FIG. 1 depicts a schematic block diagram of one embodiment of an exhaustsensor system 10. The illustrated exhaust sensor system 10 includes asensor assembly 12, an engine 14, and an exhaust system 16. The engine14 produces exhaust which moves through the exhaust system 16. Theexhaust system 16 facilitates flow of the exhaust gases to a gas outlet18, typically for emission into the atmosphere. The sensor assembly 12is at least partially inserted into the exhaust system 16 to detect aparameter within the exhaust stream. As the gas in the exhaust system 16passes over and/or through the sensor assembly 12, the sensor assembly12 detects a condition within the exhaust by measuring chemicals ortemperature or other parameters at the sensor assembly 12, as describedherein. In one embodiment, the sensor 12 is a single cell sensor withouta separate reference cell. A single cell arrangement may be beneficialin certain applications. For example, a single cell includes a pair ofelectrodes and an electrolyte. Such a single cell system might reducethe complexity and requirements of the system as well as reduce cost ofmaterials and components. In another embodiment, the system 10 includesa reference cell (not shown) mounted outside of the exhaust stream. Thismay be beneficial in certain applications. For example, in someapplications, this may allow the system 10 to achieve a higher degree ofsensitivity with reduced margins of error. In a specific embodiment, thesensor assembly 12 includes a NO_(x) sensor to detect conditions relatedto the presence of NO and/or NO₂ within the exhaust stream. However,other embodiments may be implemented to detect other chemicals orcompositions within the exhaust stream.

The exhaust sensor system 10 also includes an electronic control module20. The electronic control module 20 includes a processor 22, anelectronic memory device 24, and an output device 26. In someembodiments, the electronic memory device 24 stores one or morereferences 28 and/or other data, as described herein. The electroniccontrol module 20 also includes an original signal generator 30 and aresponse signal detector 32.

In further embodiments, the electronic control module 20 also mayinclude a control circuit (not shown) to control some or all of theoperations of the sensor assembly 12. Alternatively, some or all of thecontrol circuit functionality may be implemented at the sensor assembly12 or at another location that is not necessarily proximate theelectronic control module 20. Additionally, in some embodiments, thecontrol circuit may control a peripheral system (not shown). Someexamples of peripheral systems that may be implemented at the sensorassembly 12 include, but are not limited to, a heater (not shown) or achemical neutralizer system (not shown). Instead of or in addition tothe chemical neutralizer system, some embodiments may include anemission control element (not shown) to neutralize other aspects of thechemicals and/or substances within the exhaust system, either upstreamor downstream from the sensor assembly 10. In other embodiments, thecontrol circuit may control peripheral systems at other locations withinthe exhaust sensor system 10.

In some embodiments, reference 28 is an algorithm into which data isentered by the processor to generate a value corresponding to somecharacteristic of the exhaust stream. In other embodiments, thereference 28 is a lookup table to correlate a sensor signal to a valuefor a characteristic of the exhaust stream. In some embodiments, thevalue corresponds to one or more concentrations of gases within theexhaust stream. In another embodiment, the value corresponds to atemperature of the exhaust stream. In other embodiments, the valuecorresponds to other characteristics of the exhaust stream.

In one embodiment, the sensor assembly 12 includes a solid-stateelectrochemical gas sensor (refer to FIG. 2). Other embodiments of thesensor assembly 12 may include different types of gas sensors.

The processor 22 communicates with the original signal generator 30 toapply an original signal to the gas sensor of the sensor assembly 12.The input waveform may be any type of symmetrical or asymmetricalalternating current (AC) input waveform. Additionally, the inputwaveform may have one or more known characteristics such as frequency,amplitude, or another similar identifier. In a particular example, theapplication of a triangular input waveform may result in highersensitivities and larger sensor signals compared with the application ofa sinusoidal input waveform. This is similar to improved results thatcan be obtained using the time-domain response compared with the phaseangle measurements using frequency-domain (i.e., impedancemetric)techniques. In some embodiments, the superior response signal fordetection of gas species using the triangular waveform in thetime-domain may be related to the wave distortions that are a result ofhow gas composition influences electrochemical reactions. Additionally,the wave distortions in the time-domain may allow for higher resolution(and thus higher sensitivities and larger sensor signals) when comparedwith measurements made in the frequency-domain (i.e., impedancemetricmethods).

In some embodiments, the original signal generator 30 is controlled togenerate and apply different types of input waveforms over time.Accordingly, one or more of the waveform characteristics, or the type ofwaveform, changes over time. Such changes may be controlled in a dynamicmanner by the processor 22 to occur abruptly or over a defined amount oftime.

In response to the input waveform applied to the sensor assembly 12, theresponse signal detector 32 detects a time-domain response that is atleast partially dependent on the input waveform and any waveformalterations resulting from the composition of the gas in the exhauststream. In other words, one or more characteristics of the gas in theexhaust stream may cause time-domain changes to the response waveformrelative to the input waveform. For example, the time-domain responsemay be influenced by the presence of NO and/or NO₂ in the gas stream.The processor 22 may identify these changes and, consequently, identifyone or more corresponding characteristics of the gas stream. In oneembodiment, the response of the solid-state electrochemical gas sensoris digitally measured as a voltage-current time differential asindicated by either the time domain zero-crossing, at another specifiednon-zero magnitude, or at some combination of magnitudes. The responseobtained by the response signal detector 32 is then presented to theprocessor 22 for further use such as data storage and/or reporting viathe output device 26.

In addition to detecting the presence and/or concentration of gasspecies within the gas stream, some embodiments facilitate detection oftemperature fluctuations of the gas stream and/or sensor assembly 12.Certain input waveforms may result in response waveforms that areprimarily or solely responsive to temperature changes and do not respond(or only respond trivially) to changes in gas concentration. Forexample, a low-amplitude, high-frequency sine wave has reducedsensitivity to gas species but measureable sensitivity to temperature.Other wave forms with other characteristics may be more or lesssensitive to gas species and temperatures. For example, a trianglewaveform is capable of providing a measurement of temperature as well asdemonstrating a sensitivity to gas species. Such a waveform may begenerated intermittently to measure temperature and adjust the inputwaveform. In some embodiments, adjustment of the input waveform mayimprove the accuracy of the measurement of certain gas species. Forexample, some testing has shown as low as a 1.6 ppm margin of error inmeasurement of NO_(x) concentrations around 200 ppm and 10% O₂.

In some embodiments, the processor 22 may implement feedback from theresponse signal detector 32 to the original signal generator 30. Forexample, the processor 22 may direct the original signal generator 30 tomodify the frequency and/or shape of the input waveform to identifytemperature fluctuations directly so that the measured fluctuationscould then be used to adjust the overall sensor signal and improveaccuracy.

In further embodiments, the detection of temperature may be sequentiallyarranged with the detection of gas species. For example, a triangularinput waveform may be used to detect gas species, and then a sinusoidalinput waveform with higher frequency and lower amplitude may be used todetermine temperature. In a more specific example, the signal for NO_(x)is obtained using a triangular input waveform with 100 mV amplitude and50 Hz frequency. Then the triangular input waveform is brieflyinterrupted with a signal that can be used to identify temperaturevariations using a sine waveform with 50 mV amplitude and 10 kHzfrequency. In other embodiments, other types of simultaneous and/orsequential measurement schemes may be employed to measure any number ofcharacteristics of the gas stream.

FIG. 2A depicts a schematic diagram of one embodiment of a gas sensor 40for use in the sensor assembly of FIG. 1. The illustrated gas sensor 40includes a substrate 42, a plurality of electrodes 44 and 46, and anelectrolyte 48. In some embodiments, at least one of the plurality ofelectrodes 44 and 46 includes a sensing material such as pure gold,gold-based alloys, or doped lanthanum-based perovskites (such asstrontium-doped lanthanum manganite [LSM]). In other embodiments, one ofthe plurality of electrodes 44 and 46 includes one of the materialslisted above while the other of the plurality of electrodes 44 and 46includes a non-sensing material (such as platinum, platinum-basedalloys, or other electrically conductive metal alloy compositions). Insome embodiments, the electrolyte 48 includes an ionically conductingmaterial. For example, the electrolyte 48 may include yttria-stabilizedzirconia (YSZ). In some embodiments, the substrate 42 is an electricallyinsulating material. For example, the substrate 42 may include alumina(Al₂O₃). In some embodiments, the substrate 42 contains an embeddedresistive heater with a Pt-based allow composition. For convenienceelectrical leads are not shown.

In some embodiments, the sensor design may be symmetric with theelectrodes 44 and 46 made of the same material or substantially the samematerial. In other embodiments, the sensor design may be asymmetric withthe electrodes 44 and 46 made of different materials. Although thetime-domain methods described herein are effective for symmetric sensordesigns and geometries, in some embodiments the wave distortions in thetime-domain are more pronounced and exaggerated for sensor designs andgeometries involving asymmetric electrodes.

For example, the asymmetric electrodes may lead to large differences inreaction rates at each of the electrodes in the presence of oxygen andNO_(x). In particular, when platinum (a good oxygen catalyst with fastreaction rates) is used as one electrode, and gold is used as the otherelectrode, the wave distortions may be more pronounced than with asymmetric design or with some other asymmetric materials. In someembodiments, the asymmetric wave distortions also allow larger sensorsignals to be extracted at specified points beside the typicalzero-crossing. Thus, the choice of the specific point of magnitude toproduce the larger signal may rely on the specific sensor design andgeometry for measurements of a particular composition.

In some embodiments, an asymmetric sensor design used within the digitaltime-domain can also facilitate simultaneous measurement of multiple gasspecies. As an example, the response waveform may have asymmetricalproperties which can be attributed to two electrodes with differentcompositions (e.g., platinum and gold). The asymmetrical properties ofthe response waveform may lead to portions of the waveform that aredominated by one gas species compared to other portions of the waveformthat have a weak dependence. In this way, specific regions of thewaveform with large differences in response to a specific gas can beused to emphasize and/or isolate the contribution of the gas species.For example, in some embodiments the oxygen response can be isolatedfrom the changes in NO_(x) by “tuning” the measurement to differentregions of the waveform that exhibit either a very large response or aminimal response to oxygen as compared to NO_(x). As used herein, tuningrefers to identifying characteristics associated with a particular timeand/or magnitude of the waveform. Therefore, the asymmetric signals andmultiple voltage-current time differentials can be used to extractmeasurements of multiple gas species simultaneously. In furtherembodiments, other gas species that are potential interferents, such asammonia, also may be detected using this same methodology.

FIG. 2B depicts multiple embodiments of sensor arrangements. Thedepicted embodiments include several arrangements and material optionsfor various components. In particular, some of the embodiments includean YSZ electrolyte 48, dense strontium-doped lanthanum manganite (LSM)electrodes 44 a and 46 a, and an alumina substrate 42. Some embodimentsinclude asymmetric combinations of electrodes by combining Pt electrode44 b with dense LSM electrode 46 a. Other embodiments include gold-basedalloy electrodes 44 c and 46 b. Another embodiment includes gold wireelectrodes 46 c with platinum electrodes 44 b. Other embodiments mayinclude other arrangements and material selections for each component.

FIG. 3A depicts one embodiment of a waveform diagram 50 of an originaltriangular input waveform 52 and corresponding response waveform regions54 and 56. The input waveform 52 is representative of the input waveformgenerated by the input waveform generator 30 and applied to the sensorassembly 12. The response waveform regions 54 and 56 are representativeof response waveforms detected by the response waveform detector 32. Inone embodiment, a voltage is input to the sensor 12 and a current isreturned and measured. Other forms of original and affected signals arealso useful.

In depicted embodiment, the input waveform 52 is a voltage signalmeasured in Volts as shown on the right-hand vertical axis. The affectedwaveform with regions 54 and 56 is superimposed over the original inputwaveform 52 and is a current signal measured in nA as shown on theleft-hand vertical axis. Each waveform is plotted relative to time asshown on the horizontal axis.

In this example, the response waveform regions 54 and 56 representcurrent measured through the cell. The current oscillates back and forthcentered at a zero. The change in the general curve at the responsewaveform region 54 corresponds to a sensitivity to O₂ while the responsewaveform region 56 corresponds to a sensitivity to O₂ and NO. Withoutthe O₂ sensitivity at region 56, the curve would more closely resemblethe falling portion of the waveform without the flattened portion atregion 56. Other portions of the waveform may demonstrate sensitivitiesto other species of gas.

FIG. 3B depicts similar waveforms to the waveforms of FIG. 3A withadditional markings 58 and 60 to indicate time gaps between 0 A and 0Von both sides of the waveform. The plot of FIG. 3B has an offset of5.1×V for sensor ground. The markings 58 and 60 correspond to thecombined effect of NO_(x) and O₂, which is not evenly distributed overthe entire waveform. By measuring the time gaps between the originalsignal 52 and the affected signal with regions 54 and 56 on both therising and falling sides of the waveform and combining them, it ispossible to get a much bigger signal for the gas species as comparedwith conventional phase-shift measurements.

Although FIGS. 3A and 3B use triangular waveforms, other embodiments mayuse other types of waveforms. For example, similar results may beachieved using sinusoidal waveforms. However, in some embodiments, theresponse may be smaller or a different magnitude compared with theresponse obtained using triangular waveforms.

FIG. 3C depicts the waveforms of FIG. 3B with an emphasis 62 on aportion of the triangular waveforms. The emphasized 62 regiondemonstrates a sensitivity to O₂. The sensitivity is demonstrated by theflattening of the curve in the emphasized 62 region. This results from areversal in the polarity of the affected signal due to the presence ofO₂ (discussed further below with reference to FIG. 3D).

FIG. 3D depicts the waveforms of FIG. 3B with the markings 58 and 60 ofFIG. 3B and additional markings 64 and 66 at non-zero crossings. Bymeasuring the differential signals in two places, it can be seen thatthe effect and direction of the NO response is the same for allmeasurements, but the effect and direction of the O₂ response is toreverse polarity between measurements 58 and 64. In one example, thevoltage-current time differential signals are obtained at the zerocrossings (see markings 58 and 60) and at approximately 60% of the peakvoltage in one polarity (see markings 64 and 66). When the signals arecalibrated, the O₂ signals detected at 60% (64 and 66) can be calculatedand then subtracted out of the cumulative measurement taken at the zerocrossing (58 and 60). The remaining signal can be used to extract NO.

FIG. 3E depicts a sawtooth input waveform 52 with corresponding responsewaveform regions 54 and 56. Each of regions 54 and 56 are similar toregions 54 and 56 as shown in FIG. 3A. However, FIG. 3E illustrates theresponse regions as generated with input 52 as a sawtooth waveform. Inthe illustrated embodiment, the sawtooth waveform exhibits aninteresting phenomenon when the current leads the voltage at responseregion 56. The reversal in polarity due to the presence of O₂ in theexhaust stream pushes the current ahead enough to actually lead voltagein the time domain. This further illustrates how certain waveformsdemonstrate sensitivities, at certain regions, to certain gas specieswithin the exhaust stream.

In further embodiments, it is possible to extract gas speciesmeasurements based on the response waveform resulting from alternatinginput waveforms. For example, by using an asymmetric sawtooth inputwaveform, and alternating the direction of the asymmetry, the dominantgas species effect can be measured separately with each alternatingwaveform. In some embodiments, the use of asymmetric, alternating inputwaveforms may yield more accurate measurements for certain gas species.Measurements for temperature and other characteristics of the exhauststream may also be taken by alternating input waveforms specific tomeasurement of that species or characteristic.

FIG. 3F depicts an embodiment of response signals corresponding tovaried gas species concentrations. While FIGS. 3A-E depict resultsgathered from steady-state experiments, FIG. 3F demonstrates resultsgathered with variations in gas species concentrations. FIG. 3Fillustrates the effect of NO and O₂ on the sensor output. In particular,NO appears to constantly move the signal downward while O₂ hastendencies to reverse the polarity of the signal in certain regions.

FIG. 4 depicts one embodiment of a flow chart diagram of a method 70 formeasuring gas species using voltage-current differential in thetime-domain. The method includes identifying 72 an affected region of anaffected waveform signal corresponding to at least one characteristic ofthe gas stream. The method 70 also includes calculating 74 a differencebetween the affected region of the affected waveform signal and acorresponding region of an original waveform signal, wherein theaffected region and the corresponding region of the waveform signalshave a sensitivity specific to the at least one characteristic of thegas stream. The method 70 also includes generating 76 a value for the atleast one characteristic of the gas stream based on the calculateddifference.

FIG. 5 depicts one embodiment of a flow chart diagram of a method 80 formeasuring temperature using voltage-current differential in thetime-domain. The method includes identifying 82 a temperature parameterof an affected waveform signal. The method also includes calculating 84a change in the temperature parameter by comparing the affected waveformsignal with an original waveform signal. The method also includesgenerating 86 a value from the calculated change which corresponds tothe temperature of the gas stream.

It should be noted that at least some of the operations for the methodsmay be implemented using software instructions stored on a computeruseable storage medium for execution by a computer. As an example, anembodiment of a computer program product includes a computer useablestorage medium to store a computer readable program that, when executedon a computer, causes the computer to perform operations, including anoperation to detect an affected waveform and compare the affectedwaveform against an original waveform to determine at least onecharacteristic of a gas stream.

Some embodiments described herein may include at least one processingdevice coupled directly or indirectly to memory elements through asystem bus such as a data, address, and/or control bus. The memoryelements can include local memory employed during actual execution ofthe program code, bulk storage, and cache memories which providetemporary storage of at least some program code in order to reduce thenumber of times code must be retrieved from bulk storage duringexecution.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operations may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be implemented in anintermittent and/or alternating manner.

Embodiments of the invention can take the form of an entirely hardwareembodiment or an embodiment containing both hardware and softwareelements. In one embodiment, the invention is implemented in software,which includes but is not limited to firmware, resident software,microcode, etc. on a hardware device such as a processor, a memorydevice, or another device capable of storing non-transient signalsand/or processing related signals.

Furthermore, embodiments of the invention can take the form of acomputer program product accessible from a computer-usable orcomputer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system. For thepurposes of this description, a computer-usable or computer readablemedium can be any apparatus that can contain, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device.

The computer-useable or computer-readable medium can be an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system(or apparatus or device), or a propagation medium. Examples of acomputer-readable medium include a semiconductor or solid state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disk, and an opticaldisk. Current examples of optical disks include a compact disk with readonly memory (CD-ROM), a compact disk with read/write (CD-R/W), and adigital video disk (DVD).

Input/output or I/O devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers. Additionally, networkadapters also may be coupled to the system to enable the data processingsystem to become coupled to other data processing systems or remoteprinters or storage devices through intervening private or publicnetworks. Modems, cable modems, and Ethernet cards are just a few of thecurrently available types of network adapters.

In the above description, specific details of various embodiments areprovided. However, some embodiments may be practiced with less than allof these specific details. In other instances, certain methods,procedures, components, structures, and/or functions are described in nomore detail than to enable the various embodiments of the invention, forthe sake of brevity and clarity.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The scope of theinvention is to be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method for analysis of a gas stream,comprising: generating, by a signal generator, an original waveform;receiving, by a sensor, the original waveform; generating, by thesensor, an affected waveform signal representing at least onecharacteristic of a gas stream from the original waveform; identifying,by a processor, an affected region of the affected waveform signalcorresponding to the at least one characteristic of the gas stream bycomparing at least one portion of the affected waveform and the originalwaveform; calculating, by the processor, a voltage-current timedifferential between the affected region of the affected waveform signaland a corresponding region of an original waveform signal, wherein theaffected region and the corresponding region of the waveform signalshave a sensitivity specific to the at least one characteristic of thegas stream; generating, by the processor, a value corresponding to theat least one characteristic of the gas stream based on the calculatedvoltage-current time differential; and outputting, by the processor, thegenerated value.
 2. The method of claim 1, further comprising: directingthe original waveform signal to the sensor, wherein the sensor isdisposed at least partially within the gas stream; and detecting, by theprocessor, the affected waveform signal returned from the sensor,wherein the value is output at the sensor.
 3. The method of claim 1,further comprising determining at least two values corresponding to atleast two characteristics of the gas stream based on at least twoseparate regions of the affected waveform signal relative to theoriginal waveform signal.
 4. The method of claim 3, wherein the at leasttwo characteristics of the gas stream comprise a concentration of NO andO₂.
 5. The method of claim 2, wherein the sensor comprises atwo-electrode sensor.
 6. The method of claim 5, wherein the electrodesof the sensor are asymmetric.
 7. The method of claim 6, wherein theasymmetric electrodes comprise a first electrode comprising gold (Au)and a second electrode comprising platinum (Pt).
 8. The method of claim1, further comprising intermittently generating a temperature detectionwaveform to determine a temperature of the gas stream.
 9. The method ofclaim 8, further comprising generating a feedback signal to modify theoriginal waveform signal based on the temperature determination.
 10. Asystem for analysis of a gas stream, comprising: a signal generatorconfigured to generate an original waveform; a sensor configured to:receive the original waveform; and generate an affected waveform signalrepresenting at least one characteristic of a gas stream from theoriginal waveform; and a processor configured to: identify an affectedregion of the generated affected waveform signal corresponding to the atleast one characteristic of the gas stream by comparing at least oneportion of the affected waveform and the original waveform; calculate avoltage-current time differential between the affected region of theaffected waveform signal and a corresponding region of an originalwaveform signal, wherein the affected region and the correspondingregion of the waveform signals have a sensitivity specific to the atleast one characteristic of the gas stream; generate a valuecorresponding to the at least one characteristic of the gas stream basedon the calculated voltage-current time differential; and output thegenerated value.
 11. The system of claim 10, wherein: the signalgenerator is further configured to direct the original waveform signalto the sensor; the sensor is disposed at least partially within the gasstream; the processor is further configured to detect the affectedwaveform signal returned from the sensor; and the value is output at thesensor.
 12. The system of claim 10, wherein the processor is furtherconfigured to determine at least two values corresponding to at leasttwo characteristics of the gas stream based on at least two separateregions of the affected waveform signal relative to the originalwaveform signal.
 13. The system of claim 12, wherein the at least twocharacteristics of the gas stream comprise a concentration of NO and O₂.14. The system of claim 11, wherein the sensor comprises a two-electrodesensor.
 15. The system of claim 14, wherein the electrodes of the sensorare asymmetric.
 16. The system of claim 15, wherein the asymmetricelectrodes comprise a first electrode comprising gold (Au) and a secondelectrode comprising platinum (Pt).
 17. The system of claim 10, whereinthe processor is further configured to intermittently generate atemperature detection waveform to determine a temperature of the gasstream.
 18. The system of claim 17, wherein the processor is furtherconfigured to generate a feedback signal to modify the original waveformsignal based on the temperature determination.